We live in a semi-detached house with bay fronted windows upstairs & down (5 window bays). We recently had the old wooden windows replaced with new UPVC ones. The work was done by a reputable company and the bays were fully supported with steel rods during the work. However, I now notice a small hairline crack on either side of the upstairs bay window. running from below the window sill on either first window bay, to the skirting board below. There are no cracks on the downstairs bay, nor anywhere else in the house.

Is this normal after replacement windows? The cracks look quite small and I can't see any cracks on the outside wall.

There used to be problems with cracking to the upper bay(if built in masonry) when timber bay windows were replaced with plastic to the extent that sometimes the windows would have to be removed and bay poles and steel plates put in.
The better quality modern ones are said to have enough reinforcement to prevent this. As Alastair says some hairline cracks are quite normal.
I know one surveyor who isn't too keen on the idea of replacing timber bays with plastic when the upper bay is built in masonry.

give the new bays some time to settle then widen the cracks a little and fill with decorators caulk, let it set and then give it a light sanding before painting.

Sorry to bump an old thread but as opposed to just caulking I understand you can chisel the crack out and fit a steel strap across before plastering over. Does anybody know the correct name of these bay window crack-straps (for want of a better phrase!) and where I can get some from. Googled multiple variations and can't find an actual name of them in order to purchase.